This application relates generally to gas turbine engines and, more particularly, to gas turbine combustors.
Air pollution concerns worldwide have led to stricter emissions standards both domestically and internationally. Pollutant emissions from industrial gas turbines are subject to Environmental Protection Agency (EPA) standards that regulate the emission of oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO). In general, engine emissions fall into two classes: those formed because of high flame temperatures (NOx), and those formed because of low flame temperatures which do not allow completion of the fuel-air reaction (HC & CO).
At least some known gas turbine combustors include a plurality of mixers, which mix high velocity air with liquid or gaseous fuels prior to the mixture being ignited. Such mixers usually include a single fuel injector located at a center of a swirler which swirls incoming air to facilitate enhancing flame stabilization and mixing. Both the fuel injector and mixer are coupled to a combustor dome.
At least some known gas turbine engine combustors operate with a fuel to air ratio in the mixer that is fuel-rich, wherein additional air is added through discrete dilution holes prior to the combustion gases exiting the combustor. However, poor mixing and hot spots may occur both at the dome, where the injected fuel must vaporize and mix prior to burning, and in the vicinity of the dilution holes, wherein additional air is added to the rich dome mixture. Other known gas turbine engines use dry-low-emissions (DLE) combustors that create fuel-lean mixtures in the mixer. Because the fuel-air mixture throughout the combustor is lean, DLE combustors typically do not have dilution holes.
In operation, combustion acoustics may limit the operational range of lean premixed gas turbine combustors. To facilitate reducing combustion acoustics, at least some known gas turbine engines utilize mismatched flame temperatures. However, mismatching the flame temperatures may result in increasing NOx emissions. Other known gas turbine engines use a variety of passive means to facilitate reducing the amplitude of the combustion acoustics. For example, at least one known gas turbine engine uses a plurality of quarter-wave acoustic damper tubes to reduce combustor acoustics. Quarter-wave damper tubes operate over a relatively narrow band of frequencies, and are fabricated in a plurality of lengths. To determine the optimum length of a damper tube, a time consuming process may be required. The process includes coupling a damper tube having a predetermined length to the gas turbine, and measuring the resultant combustor acoustics. The process must generally be repeated until the optimal damper tube length has been identified.